This invention relates to a method of operating a laser and to laser apparatus making use of intra-cavity beam shaping.
Laser beam shaping tools have matured over the past few decades to allow external (to the laser cavity) reshaping of a laser beam to a desired transverse profile4. The procedures for determining the desired optical transformation are well known4, and may be implemented by a variety of methods, for example, by diffractive optical elements, free-form optics or more recently by digital holograms written to a spatial light modulator (SLM).
However, there are advantages to rather shaping the light inside the laser cavity (intra-cavity laser beam shaping) and this has been a subject of interest for a number of years2,3, with several design techniques available5-10, some of which have successfully been implemented, for example using phase-only11-14, amplitude-only1-3, 15, 16, and optically-addressed liquid crystal17 optical elements for spatial mode selection. All of these techniques require custom optics, for example, a diffractive mirror or phase plate designed for a specific mode, while the optically-addressed liquid crystal approach requires external beam shaping (e.g., diffractive optics or SLMs) to address the optic, a wavefront sensor and optimization routine to iterate towards the desired phase profile, and thus results in unconventional, metre-long, cavities.
There have also been attempts at dynamic intra-cavity beam control with deformable mirrors18-23, but such elements have very limited stroke, are limited in the phase profiles that can be accommodated18,19, and thus have found little application in laser mode shaping. Rather, such mirrors have been instrumental in high power applications such as correcting mode distortions (e.g., due to thermal load) or to maximizing energy extraction and optimization of laser brightness20-23. To date no technique has been demonstrated for the on-demand selection of laser modes.
It is an object of the invention to provide a method of operating a laser and corresponding apparatus which uses intra-cavity beam control and which allows such on-demand laser mode selection.